Solid Polymer Electrolyte and Process For Making Same

ABSTRACT

A solid polymer electrolyte membrane having a first surface and a second surface opposite the first surface, where the solid polymer electrolyte membrane has a failure force greater than about 115 grams and comprises a composite membrane consisting essentially of (a) at least one expanded PTFE membrane having a porous microstructure of polymeric fibrils, and (b) at least one ion exchange material impregnated throughout the porous microstructure of the expanded PTFE membrane so as to render an interior volume of the expanded PTFE membrane substantially occlusive; (c) at least one substantially occlusive, electronically insulating first composite layer interposed between the expanded PTFE membrane and the first surface, the first composite layer comprising a plurality of first carbon particles supporting a catalyst comprising platinum and an ion exchange material, wherein a plurality of the first carbon particles has a particle size less than about 75 nm, or less than about 50 nm, or less than about 25 nm.

RELATED APPLICATION

The present application is a divisional application of pending U.S.patent application Ser. No. 12/633,835 filed Dec. 9, 2009 and furtherclaims benefit to pending U.S. patent application Ser. No. 11/235,478filed Sep. 26, 2005.

FIELD OF THE INVENTION

The present invention relates to a solid polymer electrolyte and processfor making it, as well as its use in a catalyst coated membrane and inpolymer electrolyte membrane fuel cells.

BACKGROUND OF THE INVENTION

Fuel cells are devices that convert fluid streams containing a fuel, forexample hydrogen, and an oxidizing species, for example, oxygen or air,to electricity, heat and reaction products. Such devices comprise ananode, where the fuel is provided; a cathode, where the oxidizingspecies is provided; and an electrolyte separating the two. Theanode-electrolyte-cathode body is called the catalyst coated membraneherein. The fuel and/or oxidant typically is a liquid or gaseousmaterial. The electrolyte is an electronic insulator that separates thefuel and oxidant. It provides an ionic pathway for the ions to movebetween the anode, where the ions are produced by reaction of the fuel,to the cathode, where they are used to produce the product. Theelectrons produced during formation of the ions are used in an externalcircuit, thus producing electricity. As used herein, fuel cells mayinclude a single cell comprising only one anode, one cathode and anelectrolyte interposed therebetween, or multiple cells assembled in astack. In the latter case there are multiple separate anode and cathodeareas wherein each anode and cathode area is separated by anelectrolyte. The individual anode and cathode areas in such a stack areeach fed fuel and oxidant, respectively, and may be connected in anycombination of series or parallel external connections to provide power.

Additional components in a single cell or in a fuel cell stack mayoptionally include means to distribute the reactants across the anodeand cathode, including, but not limited to porous gas diffusion media.Various sealing materials used to prohibit mixing of the various speciesmay also be used. As used herein, the membrane electrode assembly (MEA)comprises the catalyst coated membrane and such gas diffusion media andsealing materials. Additionally, so-called bipolar plates, which areplates with channels to distribute the reactant may also be present.

A Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a type of fuel cellwhere the electrolyte is a polymer electrolyte. Other types of fuelcells include Solid Oxide Fuel Cells (SOFC), Molten Carbonate Fuel Cells(MCFC), Phosphoric Acid Fuel Cells (PAFC), etc. As with anyelectrochemical device that operates using fluid reactants, uniquechallenges exist for achieving both high performance and long operatingtimes. In order to achieve high performance it is necessary to reducethe electrical and ionic resistance of components within the device.Recent advances in the polymer electrolyte membranes have enabledsignificant improvements in the power density of PEMFCs. Steady progresshas been made in various other aspects including lowering Pt loading,extending membrane life, and achieving high performance at differentoperating conditions. However, many technical challenges are stillahead. One of them is for the membrane electrode assembly to meet thelifetime requirements for various potential applications. These rangefrom hundreds of hours for portable applications to 5,000 hours orlonger for automotive applications to 40,000 hours or longer instationary applications.

Although all of the materials in the fuel cell may be subject todegradation during operation, the integrity and health of the membraneis particularly important. Should the membrane degrade during fuel celloperation, it tends to become thinner and weaker, thus making it morelikely to develop holes or tears. Should this occur, the oxidizing gasand fuel may mix internally potentially leading to internal reactions.Because such an internal reactions may ultimately cause damage to theentire system, the fuel cell must be shut down, One well known approachto assessing the health of a membrane is to measure the amount offluoride ions in the product water of the fuel cell. Higher values ofthis so-called fluoride release rate are indicative of more attack ofthe membrane, and therefore are associated with membranes that havelower durability. Correspondingly, lower fluoride release rates areindicative of a healthier membrane, one more likely to have longer life.

As is well known in the art, decreasing the thickness of the polymerelectrolyte membrane can reduce the membrane ionic resistance, thusincreasing fuel cell power density. However, reducing the membranesphysical thickness can increase the susceptibility to damage from otherdevice components leading to shorter cell lifetimes. Variousimprovements have been developed to mitigate this problem. For example,US Pat. No. RE 37,307, US Pat. No. RE37,701, US Application No.2004/0045814 to Bahar et al., and U.S. Pat. No. 6,613,203 to Hobson, et.al. show that a polymer electrolyte membrane reinforced with a fullyimpregnated microporous membrane has advantageous mechanical properties.Although this approach is successful in improving cell performance andincreasing lifetimes, even longer life would be even more desirable.

Various approaches have been used in the art in further attempts toextend membrane life. Shortly after the development of polymermembranes, many practitioners realized that degradation of the membraneoccurred through the generation of radical species, for example, peroxyor hydroxy radicals in or near the electrodes. These very active speciesattacked the polymer and chemically degraded it. Therefore, approachesto reduce or remove these radical species have been developed. Forexample, it was recognized in the '70s, that “for applications wheremaximum performance and life are needed, the membrane is treated bydepositing a small quantity of catalyst within the solid polymerelectrolyte (SPE). The finely divided catalyst, which forms adiscontinuous layer, decomposes the small quantity of potentiallyharmful peroxy degradation species. Also there is a more intimateelectrode/electrolyte contact which leads to some performanceimprovement. The use of the catalyst within the SPE appears to increasemembrane life by an order of magnitude compared to untreated material.”[LaConti, et. al., Proceedings of the Symp. On Electrode Materials forEnergy Conversion & Storage, McIntyre, J D E; Srinivasan, S; and Will, GG; (eds), The Electrochemcal Society, Vol. 77-6, 1977, pg. 354]. Variousapproaches to achieve such compositions were subsequently developed, forexample U.S. Pat. No. 4,959,132 to Fedkiw, U.S. Pat. No. 5,342,494 toShane, et. al., U.S. Pat. No. 5,472,799 to Watanabe et. al, and U.S.Pat. No. 5,800,938 also to Watanabe.

In '132 a process for producing an in situ metallic electrocatalyticfilm proximate the surface of a solid polymer electrolyte membrane toform a composite structure useful in promoting electrochemical reactionsin fuel cells, sensors, chloralkali processes, dialysis, orelectrochemical synthesis cells is described. The method comprises thesteps of: loading metal ions into the ionomer matrix of a solid polymerelectrolyte membrane to achieve a loading level of metal ions sufficientfor forming an electronically coherent film of metal within the ionomermatrix, said metal ions being selected as those which will constitutethe chemical composition of the electrocatalytic film; and exposing atleast one face of the metal-ion-loaded membrane to a chemical reductantunder controlled conditions of time and temperature sufficient to causethe metal ions in the membrane to diffuse towards the exposed face andto be reduced to the metal (0) state while within the membrane, and toproduce in situ within the ionomer matrix of the membrane anelectronically coherent porous film of metal located predominatelywithin the membrane and near its surface, the electronically coherentfilm being comprised of metal particles in electrical contact with oneanother. Although the process described in '132 does describe a processto form an electrocatalytic film proximate the surface of a solidpolymer electrolyte membrane, it is a porous film in the membrane, andtherefore is less useful in reducing cross-over of hydrogen through themembrane. Furthermore, only unsupported electrocatalyst metal ions aredescribed.

In '494, another method for forming a catalyst impregnated fluorocarbonion exchange membrane is described. It comprises the steps of: (a)conditioning the ion exchange membrane by exchanging hydrogen ions inthe membrane with replacement cations; (b) exchanging said replacementcations with platinum catalyst ions; (c) reducing said catalyst ions toplatinum metal; (d) repeating steps “a” through “c” at least once toform a multiply impregnated membrane; and (e) exchanging any remainingreplacement cations in said multiply impregnated membrane with hydrogenand (f) equilibrating said multiply impregnated membrane wherein theplatinum metal is present in the form of discrete and isolated particleswithin the membrane. This patent involves multiple complex steps, andproduces discrete and isolated platinum metal particles that are notsupported.

In '799, a solid polymer electrolyte fuel cell is described. Itcomprises a cathode current collector, a cathode connected to thecathode current collector, an ion exchange membrane having a catalystlayer; an anode and an anode current collector connected to the anode,the catalyst layer being electronically insulated from the currentcollectors. This catalyst layer is produced by dipping in an aqueoussolution of a platinum amino salt to ion-exchange the exchange groups ofthe ion exchange resin in the electrodes with the platinum cation, andthen the catalyst metal is supported in the vicinity of the surface byreducing the platinum ion by means of such a reducing agent ashydrazine. [col 1, Ins. 62-67]. Only unsupported platinum metalcatalysts are described, and the catalyst layer is separated from thecathode by an intervening layer of ion exchange membrane [FIG. 2]. In alater issued patents, U.S. Pat. No. 5,766,787 to the same author, asolid polymer composition comprising solid polymer electrolyte selectedfrom cation exchange resin and anion exchange resin, and 0.01 to 80% inweight of at least one metal catalyst selected from the group consistingof platinum, gold, palladium, rhodium, iridium and ruthenium based onthe weight of the solid polymer electrolyte contained in the said solidpolymer electrolyte is claimed. This patent also only describesunsupported precious metal catalysts in the solid polymer electrolyte,and discloses a similar process as used in '799 to produce them.

In '938, a sandwich-type solid polymer electrolyte fuel cell is claimed.In this patent, a platinum layer (i.e. reaction catalyst layer, 7 inFIG. 2 of '938) was formed by means of sputtering onto a hydrocarbon ionexchange membrane on the anode side having a thickness of 50 microns andan EW value of 900. A commercially available perfluorocarbon-type ionexchange resin solution (“Nafion” solution) was applied on the catalystlayer on the anode side of the ion exchange membrane and dried at 60degrees C. to form an ion exchange membrane having a catalyst layerwhose total thickness was 60 microns [col. 6, Ins. 42-52]. Additionally,it is disclosed that a catalyst metal particle (29 in FIG. 4 of '938)can be present in the ion exchange resin (27 or FIG. 4 of '938) of thecathode (24 of FIG. 4 of '938). The latter embodiment (FIG. 4 of '938)only has unsupported metal catalyst particles in the cathode, while theformer embodiment (FIG. 2 of '938) discloses only unsupported metalcatalyst particles in a layer separated from the cathode by an ionexchange resin (8 in FIG. 2 of '938).

A similar approach is disclosed in U.S. Pat. No. 6,630,263 to McElroyet. al. In this patent, a fuel cell is described, comprising: a cathodeflow field plate; an anode flow field plate; an anode catalyst; acathode catalyst; and a proton exchange membrane. The proton exchangemembrane, comprises a catalyst material; and a proton exchange material,wherein the catalyst material is incorporated in the proton exchangematerial, the cathode catalyst is between the proton exchange membraneand the cathode flow field plate, the proton exchange membrane isbetween the cathode and anode catalysts, and a planar area of thecathode catalyst is from about 90% to about 99.9% of a planar area ofthe anode catalyst. In this application, the catalyst material is “ametal or an alloy, such as platinum or platinum containing alloy” [Col4, line 61-62], and the importance of using a cathode catalyst areasmaller than the anode catalyst area is taught. The concept of using asupported catalyst is not disclosed. Although the use of a reinforcementin the proton exchange membrane is disclosed [FIG. 4], the importance ofa strong solid polymer electrolyte in combination with the presence of asupported catalyst in the solid polymer electrolyte is not described.

In yet another similar approach U.S. Patent Application 20050175886 toFukuda, et. al. describes a process for producing an active solidpolymer electrolyte membrane comprising: immersing an electrolytemembrane element into a mixture of a noble metal complex solution and atleast one additive selected from a water-soluble organic solvent, anonionic surfactant and a non-metallic base to conduct anion-exchanging; washing the electrolyte membrane element with purewater; subjecting the electrolyte membrane element to a reducingtreatment; washing the electrolyte membrane element with pure water; anddrying the electrolyte membrane element; wherein the active solidpolymer electrolyte membrane comprises a solid polymer electrolyteelement, and a plurality of noble metal catalyst grains which arecarried by an ion exchange in a surface layer located inside a surfaceof said solid polymer electrolyte element and which are disperseduniformly in the entire surface layer, said surface layer having athickness t₂ equal to or smaller than 10 microns, wherein an amount CAof noble metal catalyst grains carried is in a range of 0.02mg/cm².≦CA≦0.14 mg/cm². The method described in '886 the surface layeras “noble metal catalyst grains” [col 2, ln, 1]. Further, the methodembodied in the claims is not capable of producing supported catalysts,which are present within the current invention.

In addition to the approaches described above, others have describedalternative approaches to modifying the membrane in solid polymerelectrolyte fuel cells. These include U.S. Pat. No. 6,335,442 toAsukabe, et. al., JP 2001-118591 to Morimoto, et. al., US PatentApplication 2003/0008196 to Wessel, et. al., and European PatentApplication EP 1289041 A2 to Iwassaki et. al, In these applications,solid polymer electrolytes comprising various alternatives to preciousmetal catalysts are claimed. For example, in '442 a solid polymerelectrolyte membrane comprising oxide catalysts and macrocyclic metalcomplex catalysts is claimed. Similarly, in JP2001-118591transition-metal oxides are disclosed as useful catalysts in solid-statepolyelectrolytes; in 0008196 salts, oxides or organometallic complexesof group 4 elements are claimed; while in EP 1289041 antioxidantscontaining tri-valent phosphorus or sulfur are suggested. In none ofthese cases is the formation of a layer in the solid polymer electrolyteof supported precious metal catalyst disclosed, nor is the importance ofthe mechanical properties of the membrane.

More recently, additional art in US Patent Application 2004/0043283 toCippollini, et al.; US Patent Application No, 2004/0095355 to Leistra,et. al.; and US Patent Applications 2004/0224216 and 2005/0196661 toBurlatsky et. al. has published. In '43283, a membrane electrodeassembly, comprising: an anode including a hydrogen oxidation catalyst;a cathode; a membrane disposed between said anode and said cathode; anda peroxide decomposition catalyst positioned in at least one positionselected from the group consisting of said anode, said cathode, a layerbetween said anode and said membrane and a layer between said cathodeand said membrane, wherein said peroxide decomposition catalyst hasselectivity when exposed to hydrogen peroxide toward reactions whichform benign products from said hydrogen peroxide. In '95355 a method formaking membrane electrode assemblies such as those described in '43283is claimed. In '224216, a membrane electrode assembly, comprising: ananode; a cathode; a membrane disposed between the anode and the cathode;and an extended catalyzed layer between the membrane and at least oneelectrode of the anode and the cathode, the extended catalyzed layercomprising catalyst particles embedded in membrane material andincluding a plurality of particles which are electrically connected tothe at least one electrode. Similarly, in '196661, a membrane electrodeassembly, comprising: an anode; a cathode; a membrane disposed betweenthe anode and the cathode; and an extended catalyzed layer between thecathode and the membrane, the extended catalyzed layer being adapted toreduce oxygen, and decompose hydrogen peroxide and free radicals toproduce water.

In all four of these applications, a peroxide decomposition catalyst ispresent, and that catalyst either “has selectivity when exposed tohydrogen peroxide” ('95355 e.g., claims 1, 25, and Paragraphs 8; and'43283, e.g., claims 1, 10, 26, 33 and Paragraphs 8, 9 &10) or is“electrically connected to cathode” “(196661, Paragraph 23) or “to atleast one electrode” ('224216, Paragraph 10). In all four cases, thelayer is shown as part of an extended electrode (e.g., FIG. 1a in'224216, FIG. 4 in '95355 and '43283 and FIG. 3 in '196661). In '95355and '43283 the peroxide decomposition catalyst may also be disposed in aseparate layer (70 in FIG. 6 in '95355 and '43283) by being dispersedthrough the layer. In this case though, the membrane is homogeneousoutside of dispersed peroxide decomposition catalyst layer, (as shown inFIG. 6 in '95355 and'43283). Further, the critically important role ofthe mechanical properties of the membrane discovered herein is notdisclosed, nor are any specific characteristics of the dispersedperoxide decomposition catalyst disclosed.

Additional related art has focused on hydrating the membrane through theuse of various solid particles is given by U.S. Pat. No. 5,203,978 toTsou, et al.; and to Mathias et. al., in U.S. Pat. No. 6,824,909. Ineach of these an inorganic particle such as a boride, carbide or nitrideof a Groups IIIB, IVA, IVB, VB, and VIB metal ('978) or a zeolite ('909)is present. In '978 no catalyst is present, while in '909 a catalyst ispresent, but only on “adsorbent particles embedded in the membrane whichadsorb water under wet conditions” [col. 2, ln. 8-10]. Non-absorbingparticles, for example carbon, are not considered described.

In JP 2003-123777 to Takabe, et. al., a polymer electrolyte fuel cellcomprising a hydrogen ion conductive polymer electrolyte membrane, and apair of separators having gas flow channels whereby fuel gas is suppliedto and discharged from one of the electrodes, and antioxidant gas issupplied to and discharged from the other, wherein said fuel cell ischaracterized in that the electrodes are provided with catalyst layersin contact with the hydrogen ion conductive polymer electrolytemembrane, and at least one of the catalyst layers of the electrodes hasa hydrogen ion conductive polymer electrolyte, electroconductive carbonparticles that support the catalyst particles, and a peroxidedecomposition catalyst is claimed. In the specification and workingexamples of '123777, the inventors emphasize the importance ofelectrical isolation of the peroxide decomposition catalyst from theelectrode. For example, “it is also effective to electronically insulatethe space between the peroxide decomposition catalyst and theelectrodes.” (Paragraph 14), and “it is also effective for the peroxidedecomposition catalyst to be supported on electrically insulatingparticles. (also Paragraph 14)”. In fact, the inventors in '123777 go togreat lengths to provide an electrically isolated peroxide decompositioncatalyst, for example by mixing Pt/carbon catalyst with an ionomersolution followed by drying, hardening, and crushing of the mixture(Working Example 1). We have discovered, surprisingly, that suchelectrical isolation is not required to extend life and reducedegradation of solid polymer electrolytes. In this application, asdescribed more fully below, a substantially occlusive, electronicallyinsulating composite layer is present, but the individual catalyst onsupporting particles do not need to be electrically isolated from theelectrode as taught by Takabe, et. al. In fact, carbon supportparticles, which are electrically conductive, are effective in theinstant invention of this application without the additional treatmentsrequired by Takabe et. al. as described in his Working Example 1.

During normal operation of a fuel cell or stack the power densitytypically decreases as the operation time goes up. This decrease,described by various practitioners as voltage decay, fuel celldurability, or fuel cell stability, is not desirable because less usefulwork is obtained as the cell ages during use. Ultimately, the cell orstack will eventually produce so little power that it is no longeruseful at all. Furthermore, during operation the amount of fuel (e.g.,hydrogen) that crosses over from the fuel side to the oxidizing side ofthe cell will increase as the health of the membrane deteriorates. Inthis application, this hydrogen cross-over will be used to determinemembrane life.

A life test is generally performed under a given set of operatingconditions for a fixed period of time. The test is performed under aknown temperature, relative humidity, flow rate and pressure of inletgases, and is done either in fixing the current or the voltage. In thisapplication, the life tests are performed under constant currentconditions, though it is well known in the art that constant voltagelife tests will also produce decay in the power output of a cell.Herein, life is determined by temporarily stopping a life test, i.e.,removing the cell from external load, and then determining the level ofhydrogen cross-over in the cell. If the hydrogen cross-over is abovesome predetermined level, 2.5 cm³ H₂/min is used herein, then the testis ended, and the life is calculated as the number of hours the cell hasoperated. (Specific details of the test protocol used herein for lifedetermination are described below).

Although there have been many improvements to fuel cells in an effort toimprove life of fuel cells, there continues to be an unmet need for evenmore durable fuel cells, and in particular, more durable membranematerials for use in PEMFCs.

SUMMARY OF THE INVENTION

The instant invention of this application includes a solid polymerelectrolyte membrane having a first surface and a second surfaceopposite the first surface, where the solid polymer electrolyte membranehas a failure force greater than about 115 grams and comprises acomposite membrane consisting essentially of (a) at least one expandedPTFE membrane having a porous microstructure of polymeric fibrils, and(b) at least one ion exchange material impregnated throughout the porousmicrostructure of the expanded PTFE membrane so as to render an interiorvolume of the expanded PTFE membrane substantially occlusive; (c) atleast one substantially occlusive, electronically insulating firstcomposite layer interposed between the expanded PTFE membrane and thefirst surface, the first composite layer comprising a plurality of firstcarbon particles supporting a catalyst comprising platinum and an ionexchange material, wherein a plurality of the first carbon particles hasa particle size less than about 75 nm, or less than about 50 nm, or lessthan about 25 nm. Optionally, this solid polymer electrolyte may alsoinclude at least one substantially occlusive, electronically insulatingsecond layer interposed between the expanded PTFE membrane and one ofthe group consisting of the first surface, the second surface and asurface of the substantially occlusive, electronically insulating firstcomposite layer, where the substantially occlusive, electronicallyinsulating second layer is selected from the group of an ion exchangematerial, and a solid dispersion comprising a plurality of second carbonparticles supporting a catalyst comprising platinum and an ion exchangematerial, wherein a plurality of the second carbon particles has aparticle size less than about 75 nm, less than about 50 nm or less thanabout 25 nm.

An alternate embodiment of the invention is a solid polymer electrolytemembrane having a first surface and a second surface opposite the firstsurface, where the solid polymer electrolyte membrane has a failureforce greater than about 115 grams and comprises a composite membraneconsisting essentially of (a) at least one expanded PTFE membrane havinga porous microstructure of polymeric fibrils, and (b) a solid dispersioncomprising a plurality of first carbon particles supporting a catalystcomprising platinum and at least one ion exchange material, wherein aplurality of the carbon particles has a particle size less than about 75nm, less than about 50 nm or less than about 25 nm; and this soliddispersion is impregnated throughout the porous microstructure of theexpanded PTFE membrane so as to render an interior volume of theexpanded PTFE membrane substantially occlusive. Optionally, the solidpolymer electrolyte may also include at least one substantiallyocclusive, electronically insulating layer interposed between theexpanded PTFE membrane and either the first surface or the secondsurface, where the substantially occlusive, electronically insulatinglayer is selected from the group of an ion exchange material, and asolid dispersion comprising a plurality of second carbon particlessupporting a catalyst comprising platinum and an ion exchange material,wherein a plurality of the second carbon particles has a particle sizeless than about 75 nm, less than about 50 nm or less than about 25 nm.

In another embodiment, the invention includes a solid polymerelectrolyte membrane having a first surface and a second surfaceopposite the first surface, where the solid polymer electrolyte membranehas a modulus greater than about 1.75×106 g/cm2 and a thickness of lessthan about 50 microns, and comprises a composite membrane consistingessentially of (a) at least one expanded PTFE membrane having a porousmicrostructure of polymeric fibrils, and (b) at least one ion exchangematerial impregnated throughout the porous microstructure of theexpanded PTFE membrane so as to render an interior volume of theexpanded PTFE membrane substantially occlusive; (c) at least onesubstantially occlusive, electronically insulating first composite layerinterposed between the expanded PTFE membrane and the first surface,where this first composite layer comprises a plurality of first carbonparticles supporting a catalyst comprising platinum and an ion exchangematerial, wherein a plurality of the first carbon particles has aparticle size less than about 75 nm, less than about 50 nm or less thanabout 25 nm. Optionally, the solid polymer electrolyte may also includeat least one substantially occlusive, electronically insulating secondlayer interposed between the expanded PTFE membrane and one of the groupconsisting of the first surface, the second surface and a surface of thesubstantially occlusive, electronically insulating first compositelayer, where the substantially occlusive, electronically insulatingsecond layer is selected from the group of an ion exchange material, anda solid dispersion comprising a plurality of second carbon particlessupporting a catalyst comprising platinum and an ion exchange material,and a plurality of the second carbon particles has a particle size lessthan about 75 nm, less than about 50 nm or less than about 25 nm.

In another embodiment, the invention includes a solid polymerelectrolyte membrane, where the solid polymer electrolyte membrane has amodulus greater than about 1.75×10⁶ g/cm2 and a thickness of less thanabout 50 microns, and comprises a composite membrane consistingessentially of (a) at least one expanded PTFE membrane having a porousmicrostructure of polymeric fibrils, and (b) a solid dispersioncomprising a plurality of first carbon particles supporting a catalystcomprising platinum and at least one ion exchange material, wherein aplurality of the carbon particles has a particle size less than about 75nm, less than about 50 nm or less than about 25 nm; and the soliddispersion is impregnated throughout the porous microstructure of theexpanded PTFE membrane so as to render an interior volume of theexpanded PTFE membrane substantially occlusive. Alternatively, the solidpolymer electrolyte of this embodiment may also include at least onesubstantially occlusive, electronically insulating layer is interposedbetween the expanded PTFE membrane and either the first surface or thesecond surface, where the substantially occlusive, electronicallyinsulating layer is selected from the group of an ion exchange material,and a solid dispersion comprising a plurality of second carbon particlessupporting a catalyst comprising platinum and an ion exchange material,wherein a plurality of the second carbon particles has a particle sizeless than about 75 nm, less than 50 nm or less than 25 nm.

In another embodiment of the invention, a solid polymer electrolytemembrane has a first surface, and the solid polymer electrolyte membranehas a failure force greater than about 115 grams, and a thickness lessthan about 40 microns. It comprises (a) an ion exchange material and (b)a plurality of particles supporting a catalyst, where the particles aredispersed in a substantially air occlusive, electronically insulatinglayer adjacent to the first surface, and a plurality of the particlessupporting a catalyst have a size less than about 75 nm, less than 50 nmor less than 25 nm. Further, the solid polymer electrolyte may also havethe microporous polymer membrane interposed between the electronicallyinsulating layer and a second surface opposite the first surface. Thecatalyst in this embodiment may comprise a precious metal, or it maycomprise platinum; the particle supporting the catalyst may comprisecarbon, the solid polymer electrolyte may further comprises amicroporous polymer membrane, including but not limited topolytetrafluoroethylene or expanded polytetrafluoroethylene. Theconcentration of the catalyst may be less than about 5 weight percent ofthe ion exchange material, less than about 3 weight percent, less thanabout 1 weight percent or about 1 weight percent of the ion exchangematerial. Additional embodiments include any of the solid polymerelectrolyte as described in this paragraph wherein a plurality of thecatalyst particles has a size between about 1 nm and about 15 nm insize.

Yet more embodiments of the invention include catalyst coated membranesthat comprise (a) an anode comprising a catalyst for oxidizing fuel, (b)a cathode comprising a catalyst for reducing an oxidant, and a (c) asolid polymer electrolyte interposed between the anode and the cathode,where the solid polymer electrolytes comprise any of the solid polymerelectrolytes described in the proceeding five paragraphs.

Further embodiments of the invention include fuel cells comprising anyof the catalyst coated membrane described in the proceeding paragraphwherein sufficient fuel is supplied to the anode and sufficient oxidantis supplied to the cathode of the catalyst coated membrane to establisha voltage between the anode and the cathode when the anode and cathodeare electrically connected through an external load.

Yet another embodiment of the invention is a method to prepare an airocclusive integral composite membrane that comprises the steps of (a)preparing an ink solution comprising a precious metal catalyst on asupporting particle and an ion exchange material; (b) providing apolymeric support having a microstructure of micropores; (c) applyingeither the ink solution or a solution comprising an ion exchange resinto the polymeric support; (d) optionally, repeating step (c); wherein atleast one application in step (c) or (d) uses the ink solution. Theconcentration of the precious metal catalyst based on weight percent ofdry ion exchange material may be between about 0.1% and 10%, betweenabout 0.5% and 3%, about 2.5%, or about 1%. Further, step (a) mayfurther include step (a1), a step to reduce the concentration of largeparticles in the ink. Such a step may comprise filtering, or the use ofa centrifuge. Step (a) may also comprise the use of a high shear mixerto prepare the ink solution, and the high shear mixer may be amicrofluidizers, or a rotor-stator mixers comprising at least one stage.When the high shear mixer is a microfluidizer it may operate at apressure between about 1,000 and about 25,000 psi, The supportingparticle may comprises carbon in the method; the precious metal tocatalyst may comprises platinum; and the polymer support may comprisespolytetrafluoroethylene or expanded polytetrafluoroethylene.

Further embodiments of the inventive method include the method describedin the proceeding paragraph wherein step (c) further includes, (e1)applying the ink solution to a thin polymer film and (c2) applying thepolymer support having a microstructure of micropores to the inksolution on the thin polymer film. The thin polymer film comprisespolyethylene, polyethylene terephthalate polypropylene, poly vinylidenechloride, polytetrafluoroethylene, polyesters, or combinations thereof.It may also comprise a coating capable of enhancing the releasecharacteristics of the polymer film.

Yet additional embodiments of the inventive method include methodswherein step (c) further includes, a step, step (c3), of drying thesupport after each application of ion exchange material solution toremove solvent from the solution; methods wherein there is a furtherstep after step (d) of heating the air occlusive integral compositemembrane at an elevated temperature; methods wherein the elevatedtemperature is between about 100 degrees C. and about 175 degrees C.,between about 120 degrees C. and about 160 degrees C.; and methodswherein the air occlusive integral composite membrane is held at theelevated temperature for between about 1 minute and about 10 minutes, orbetween about 0.3 minutes and about 5 minutes.

Yet more embodiments of the invention include a solid polymerelectrolyte membrane having a first surface and a second surfaceopposite the first surface, where the solid polymer electrolyte membranehas a failure force greater than about 115 grams and comprises acomposite membrane consisting essentially of (a) at least one expandedPTFE membrane having a porous microstructure of polymeric fibrils, and(b) at least one ion exchange material impregnated throughout the porousmicrostructure of the expanded PTFE membrane so as to render an interiorvolume of the expanded PTFE membrane substantially occlusive; (c) atleast one substantially occlusive, electronically insulating firstcomposite layer interposed between the expanded PTFE membrane and thefirst surface, where the first composite layer comprises a plurality offirst particles supporting a catalyst comprising a precious metal and anion exchange material, wherein a plurality of the first particles has aparticle size less than about 75 nm, less than about 50 nm or less thanabout 25 nm. Optionally, the solid polymer electrolyte may also includeat least one substantially occlusive, electronically insulating secondlayer interposed between the expanded PTFE membrane and one of the groupconsisting of the first surface, the second surface and a surface of thesubstantially occlusive, electronically insulating first compositelayer, where the substantially occlusive, electronically insulatingsecond layer selected from the group of an ion exchange material, and asolid dispersion comprising a plurality of second support particlessupporting a catalyst comprising a precious metal and an ion exchangematerial, wherein a plurality of the second particles has a particlesize less than about 75 nm, less than about 50 nm or less than about 25nm. In this embodiment, the first and second particles may be selectedfrom the group consisting of silica; zeolites; and oxides and carbidesof the group IVB, VB, VIB VIIB, and VIII transition metals; andcombinations thereof; the precious metal may be selected from the groupconsisting essentially of platinum, gold, palladium, rhodium, iridium,ruthenium and combinations thereof.

In yet another embodiment of the invention, a solid polymer electrolytecomprises an ion exchange material; and a plurality of catalystparticles on a plurality of support particles; wherein a firstinterparticle spacing between the support particles is less than about600 nm. In this embodiment the first interparticle spacing may be lessthan about 450 nm, less than 300 nm, or less than 150 nm. Additionally,the solid polymer electrolyte may have a second interparticle spacingbetween the catalyst particles of less than about 50 nm, less than about30 nm, or less than about 12 nm. In that case, the catalyst material maycomprise a precious metal, the concentration of the catalyst may be lessthan or equal to about 5% by dry weight of the ion exchange material,less than or equal to about 3% by dry weight of the ion exchangematerial, less than or equal to about 1% by dry weight of the ionexchange material, or about 1% by dry weight of the ion exchangematerial. The precious metal may be selected from the group consistingessentially of gold, palladium, rhodium, iridium, ruthenium andcombinations thereof, or it may comprises platinum. In either the casewhen the precious metal comprises carbon or when it is selected from theenumerated list, the support particle may be selected from the groupconsisting essentially of silica; zeolites; and oxides and carbides ofthe group IVB, VB, VIB VIIB, and VIII transition metals; andcombinations thereof; or it may comprise carbon. Further, the supportparticle may have a size less than about 40 nm, or a size less thanabout 5 nm.

Another embodiment is a solid polymer electrolyte comprising an ionexchange material; and a plurality of catalyst particles on a pluralityof support particles; wherein an interparticle spacing between thecatalyst particles is less than about 50 nm, less than about 30 nm orless than about 12 nm. The catalyst material may comprises a preciousmetal, the concentration of the catalyst may be less than or equal toabout 5% by dry weight of the ion exchange material, less than or equalto about 3%, equal to about 2.5%, or equal to about 1% by dry weight ofthe ion exchange material. The precious metal may be selected from thegroup consisting essentially of gold, palladium, rhodium, iridium,ruthenium and combinations thereof, or it may comprise platinum. Ineither case the support particle may be selected from the groupconsisting essentially of silica; zeolites; and oxides and carbides ofthe group IVB, VB, VIB VIIB, and VIII transition metals; andcombinations thereof, or it may comprise carbon.

Additional embodiments of the invention include a catalyst coatedmembrane comprising (a) an anode comprising a catalyst for oxidizingfuel, (b) a cathode comprising a catalyst for reducing an oxidant, and a(c) a solid polymer electrolyte interposed between the anode and thecathode, where the solid polymer electrolyte comprises any of thosedescribed in the proceeding three paragraphs.

Yet more embodiments of the invention include fuel cells comprising thecatalyst coated membranes of the proceeding paragraph wherein sufficientfuel is supplied to the anode and sufficient oxidant is supplied to thecathode to establish a voltage between the anode and the cathode whenthe anode and the cathode are electrically connected through an externalload.

DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from thefollowing description when considered in conjunction with theaccompanying figure.

FIG. 1 is a drawing illustrating several embodiment of the inventivesolid polymer electrolytes.

FIG. 2 is a drawing illustrating additional embodiments of the inventivesolid polymer electrolytes.

FIG. 3 is a drawing illustrating further embodiments of the inventivesolid polymer electrolytes.

FIG. 4 is a drawing illustrating yet additional embodiments of theinventive solid polymer electrolytes.

FIG. 5 is a schematic of catalyst particles on a supporting particle.

FIG. 6 schematically illustrates an embodiment of the inventive methodfor preparing the inventive solid polymer electrolytes.

FIG. 7 is a drawing of a fuel cell that uses the inventive solid polymerelectrolyte.

FIG. 8 is a transmission electron micrograph of the material prepared inComparative Example 2.

FIG. 9 is a transmission electron micrograph of a cross-section of aportion the solid polymer electrolyte prepared in Example 8.

FIG. 10 is a higher magnification transmission electron micrograph ofthe FIG. 9.

FIG. 11 is a plot of the real versus imaginary components of the testdescribed in Example 13 and Comparative Example 7.

DETAILED DESCRIPTION OF THE INVENTION

In order to develop membranes that have a long-life in a fuel cell, themechanisms of failure need to be understood. Without being held to anyparticular theory, it is known in the art that there are two major formsof membrane failure, chemical and mechanical. The latter has beenaddressed by various approaches, for example by the formation ofcomposite membranes described by Bahar et al. in RE 37,707. Approachesto address the former have also been proposed, for example in GB1,210,794 assigned to E. I. Du Pont de Nemours, Inc., where a chemicalprocess to stabilize ionomers was described. Degradation, as observed bythe concentration of fluoride ions in various ex-situ or in-situ fuelcell tests, can thus be reduced.

The present invention involves a process for making, and a compositionof, solid polymer electrolytes that is capable of reducing electrolytedegradation as observed by fluoride release rates from operating fuelcells. Inventors have discovered a composition of solid polymerelectrolyte (SPE) that surprisingly reduces membrane degradation asobserved by fluoride release rates, and gives a concomitant increase inmembrane life. Inventors have discovered that when a plurality of verysmall particles (for example, less than about 75 nm) that are supportinga catalyst is dispersed in a substantially air occlusive, electronicallyinsulating layer, preferably in an SPE that has high strength,unexpectedly long life is observed when the SPE is tested in a fuelcell.

FIG. 1 shows a schematic of a three different embodiments of theinventive solid polymer electrolyte 10. SPE 10 typically is thin, lessthan 100 microns, preferably less than 75 microns, and most preferablyless than 40 microns thick. It comprises an ion exchange material 11that is able to conduct hydrogen ions at a high rate in typical fuelcell conditions. The ion exchange materials may include, but are notlimited to compositions comprising phenol sulfonic acid; polystyrenesulfonic acid; fluorinated-styrene sulfonic acid; perfluorinatedsulfonic acid; sulfonated Poly(aryl ether ketones); polymers comprisingphthalazinone and a phenol group, and at least one sulfonated aromaticcompound; aromatic ethers, imides, aromatic imides, hydrocarbon, orperfluorinated polymers in which ionic an acid functional group orgroups is attached to the polymer backbone. Such ionic acid functionalgroups may include, but are not limited to, sulfonic, sulfonimide orphosphonic acid groups. Additionally, the ion exchange material 11 mayfurther optionally comprise a reinforcement to form a compositemembrane. Preferably, the reinforcement is a polymeric material. Thepolymer is preferably a microporous membrane having a porousmicrostructure of polymeric fibrils, and optionally nodes. Such polymeris preferably expanded polytetrafluoroethylene, but may alternativelycomprise a polyolefin, including but not limited to polyethylene andpolypropylene. An ion exchange material is impregnated throughout themembrane, wherein the ion exchange material substantially impregnatesthe microporous membrane to render an interior volume of the membranesubstantially occlusive, substantially as described in Bahar et al,RE37,307, thereby forming the composite membrane.

The SPE 10 of FIG. 1 also comprises a plurality of particles 14supporting a catalyst, where the particles are dispersed in asubstantially air occlusive, electronically insulating layer 13 adjacentto the surface. A plurality of the particles 14 supporting a catalysthave a size less than about 75 nm, or preferably less than about 50 nm.Such particles 14 may be agglomerated together in groups of two, threeor even in larger groupings of many particles, though it is preferablethat they are separated in smaller clusters of a few particles, and mostpreferably, as individual particles. The insulating layer 13 may be onlyon one side of the ion exchange material 11 (FIG. 1 a and 1 b) or onboth sides (FIG. 1 c). Optionally, a second ion exchange material 12 mayalso be present (FIG. 1 b) on the side opposite the electronicallyinsulating layer 13. The composition of ion exchange material 12 may bethe same as ion exchange material 11, or it may be of a differentcomposition.

FIGS. 2-4 schematically illustrate alternative approaches for theinventive solid polymer electrolyte. In FIG. 2, the solid polymerelectrolyte 10 has a plurality of particles 14 supporting a catalystwithin a composite membrane 21 consisting essentially of at least oneexpanded. PTFE membrane having a porous microstructure of polymericfibrils, and (b) a solid dispersion comprising a plurality of firstcarbon particles supporting a catalyst comprising platinum and at leastone ion exchange material, wherein a plurality of the carbon particleshas a particle size less than about 75 nm, and the solid dispersion isimpregnated throughout the porous microstructure of the expanded PTFEmembrane so as to render an interior volume of the expanded PTFEmembrane substantially occlusive. Additionally, a substantially airocclusive, electronically insulating layer 13 may be adjacent to one(FIG. 2 b) or both (FIG. 2 c) surfaces. Optionally, a second ionexchange material 12 of the same, or of a different composition thanused in 21 may also be present (FIG. 1 b) on the side opposite theelectronically insulating layer 13. Alternatively, ion exchange material11, ion exchange material 12, composite membrane 21, and substantiallyair occlusive, electronically insulating layer 13 may also be present invarious alternating arrangements, some examples of which areschematically in FIG. 3 a-FIG. 3 g and FIG. 4 a-FIG. 4 e.

A schematic of the cross-section of the particles 14 supporting acatalyst used in the inventive materials is shown in FIG. 5. Theparticles 14 comprise a support material 51 onto which catalyst 52 hasbeen deposited. Support material may comprise silica; zeolites; carbon;and oxides and carbides of the group IVB, VB, VIB VIIB, and VIIItransition metals; and combinations thereof. Carbon is a particularlypreferable support material. They preferably have high surface area, andso should be small in size, less than 75 nm, or preferably less than 50nm, or less than 25 nm. They may also optionally be porous. Catalyst 52comprises metals, oxides or carbides known to be active catalyticallyfor the oxidation of active species. These include, but are not limitedto catalysts comprising precious metals, for example platinum, gold,palladium, rhodium, iridium, ruthenium and combinations thereof; andcatalytically active oxide and carbides of the group IVB, VB, VIB VIIB,and VIII transition metals; and combinations thereof. Particularlypreferable catalysts are platinum metal or platinum metal alloys. Thecatalyst 52 is small in size to maximize its surface area and increaseits effectiveness, preferably between about 1 nm and 10 nm in size.

Use of such catalysts on support particles as described herein in anymembrane reduces membrane degradation as observed by very low fluoriderelease rates during fuel cell operation. In order to achieve very longlife in a fuel cell, a combination of a high SPE strength and a layercomprising a plurality of catalyst on a supporting particle should bepresent in the electrolyte. The strength of the membrane can bequantified using several approaches known in the art, but herein, wechoose to quantify strength using a tensile test. The details aredescribed more fully below, but four parameters are extracted from thistest, the failure force, the tensile strength, the modulus and thestiffness. At least one of these must be above a critical value toachieve the very long electrolyte life described in this invention. Thesolid polymer electrolyte can achieve the high strength using any of theapproaches known in the art to improve strength in polymer films,including but not limited to, adjusting processing to prepare highstrength polymer films, for example by extrusion or stretching to orientthe polymer film; reinforcing the film with inorganic or polymerparticles; or by reinforcing with fabrics, porous or microporousinorganic or polymer films. Particularly preferably methods forpreparing a strong solid polymer electrolyte are those taught by Baharin '707, or by Hobson in '203, which use microporous ePTFE membranes toform composite electrolytes.

An inventive method for preparing an air occlusive integral compositemembrane has also been discovered. The method comprises the steps of (a)preparing an ink solution comprising a precious metal catalyst on asupporting particle and an ion exchange material; (b) providing apolymeric support having a microstructure of micropores; (c) applyingeither the ink solution or a solution comprising an ion exchange resinto the polymeric support; (d) optionally, repeating step (c); wherein atleast one application in step (c) or (d) uses the ink solution. In thisapplication, an ink is considered to be a solution containing a catalyston a supporting particle that is dispersed in a solvent. The inksolution preferably also contains an ion exchange polymer. Solvents usedin the ink are those generally known in the art, including but notlimited to alcohols, such as ethanol and propanol, or other organicsolvents. The preparation of the ink solution preferably uses a highshear mixer, where the high shear mixer may include, but is not limitedto, microfluidizers, and rotor-stator mixers comprising at least onestage. Particularly preferable high shear mixers are microfluidizerscapable of operating at pressures between 5.000 psi and 25,000 psi. Theink is preferably very well mixed, which may be accomplished by one,two, three or more passes through the high shear mixer. Theconcentration of the precious metal catalyst in the ink is between about0.1% and about 20% by dry weight of the ion exchange material, andpreferably between about 0.5% and about 3%. This ink may be prepared inone, two or more separate steps if desired. If it is prepared in two ormore steps, a more concentrated solution is made in the first step, andsubsequent steps are dilutions with ion exchange material to arrive atthe final desired concentration. When more than one step of preparingthe ink is used, the high shear mixing step described above may be usedin one or more of the ink preparation steps. If desired, the first stepin a multi-step ink preparation process may be accomplished in advanceof the succeeding steps, in which case the ink may be stored for aperiod of time. If such a concentrated ink is stored for longer thanabout 30-60 minutes, then the high shear mixing step is preferablyrepeated at least once, and more preferably two or three times beforeany subsequent dilution steps needed to arrive at the final ink used forsubsequent processing.

Additional steps to remove large agglomerates in the ink solution mayalso be performed, if desired, at any stage during the ink preparation.Such steps may include, but are not limited to, filtering and using acentrifuge. In either case, the number of large particles removed can becontrolled. In the former, by the particular filter chosen; in thelatter, by the length of time the sample is centrifuged, and/or thespeed of the centrifuge. The centrifuge speed may be varied from betweena few hundred rpm, to many thousand rpm, with the higher speeds beingpreferable. The time to centrifuge may vary from a few minutes to anhour or longer. Shorter times at higher speeds, for example less than 30minutes at 3000-5000 rpm, are preferable to reduce processing times.

The ion exchange material in the ink may be any known in the art, forexample those described above for ion exchange material 11. The preciousmetal catalyst on a supporting particle may be any of those describedabove for FIGS. 5, 52 and 51, respectively.

The polymeric support having a microstructure of micropores, may be anysuch material known in the art, including but not limited to microporouspolyethylene, polypropylene or polytetrafluoroethylene. A particularlypreferable polymeric support is expanded PTFE, such as those describedin U.S. Pat. No. 3,953,566 to Gore, in U.S. Pat. No. 6,613,203 Hobsonet. al., or in U.S. Pat. No. 5,814,405 to Branca, et. al. Preferably,the polymeric support should be sufficiently strong and/or heavy so thatthe final solid polymer electrolyte has a failure force (defined morefully below) of greater than 115 g.

The ink solution or a solution comprising an ion exchange resin may beapplied to the polymeric support using any process known in the art,including but not limited to the process described in U.S. Pat. NumberRE37,707 to Bahar et. al. Another embodiment of the method of theinvention for applying the ink to the polymeric support is shown in FIG.6. In this embodiment, an ink is applied to a thin polymer film 64 usingany means known to one of ordinary skill in the art, for example using apump, syringe 63 or such. The ink is prepared as described above, so maybe prepared in a multistep process starting with a concentrated ink thatis subsequently diluted, or directly in one step to obtain the desiredcatalyst concentration in the ink. The applied ink 62 is then spreadinto a thin layer 66 using any means known in the art for making a thinliquid layer, including but not limited to a draw bar or meyer bar,shown schematically in FIG. 6 as 61. Subsequently, the polymeric support65 having a microstructure of micropores is placed on the liquid layer66 and allowed to imbibe. The thin polymer film 64 comprisespolyethylene, polyethylene terephthalate polypropylene, poly vinylidenechloride, polytetrafluoroethylene, polyesters, or combinations thereof,and may further comprise a coating of a release material, for example afluoropolymer compound, to enhance the release of the final product fromthe polymer film. After the film is completely imbibed, it is allowed todry, and may optionally be heated to decrease the drying time. Suchheating, shown schematically in FIG. 6 as 67, may be accomplished withany means known in the art, including but not limited to forced airheaters, ovens, infrared driers and the like. The process may berepeated if desired, using the same or a different ink, or the same or adifferent ion exchange resin.

When the imbibing steps are completed, an additional heating step at anelevated temperature may optionally be applied using an oven, infraredheater, forced air heater or the like. The temperature of this heatingstep is between about 100° C. and about 175° C. and preferably betweenabout 120 degrees C. and about 160° C. The solid polymer electrolyte isheld at the elevated temperature for between about 1 minute and about 10minutes, and preferably for between about 1 minutes and about 3 minutes.Finally, the completed solid polymer electrolyte membrane is cooled, andremoved from the thin polymer film before use. The removal may beaccomplished by simply pulling the SPE off the thin polymer film, eitherin air or in water.

As is well understood by one of ordinary skill in the art, the processdescribed above and in FIG. 6 can by automated using roll goods, andautomated pay-off and collection systems so that each step isaccomplished in a continuous fashion, and the final product is a roll ofsolid polymer electrolyte supported on a thin polymer film.

The solid polymer electrolyte of the instant invention may also be usedto form a catalyst coated membrane (CCM) using any methods known in theart. In FIG. 7, the CCM 70 comprises an anode 71 of a catalyst foroxidizing fuel, a cathode 72 for reducing an oxidant, and the solidpolymer electrolyte 10 described above interposed between the anode andcathode. The anode and cathode may be prepared using any of theprocedures known in the art including but not limited to physical orchemical deposition, either on a supporting particle, or directly on theSPE, or from a catalyst-containing ink solution containing the catalyststhat is deposited either directly on the SPE, or on a film that issubsequently used in a lamination step to transfer the electrode to theSPE.

The anode and cathode electrodes comprise appropriate catalysts thatpromote the oxidation of fuel (e.g., hydrogen) and the reduction of theoxidant (e.g., oxygen or air), respectively. For example, for PEM fuelcells, anode and cathode catalysts may include, but are not limited to,pure noble metals, for example Pt, Pd or Au; as well as binary, ternaryor more complex alloys comprising the noble metals and one or moretransition metals selected from the group Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Y, Zr, Nb, Mo, Ru, Rh, Ag, Cd, In, Sn, Sb, La, Hf, Ta, W, Re, Os,Ir, Ti, Pb and Bi. Pure Pt is particularly preferred for the anode whenusing pure hydrogen as the fuel. Pt—Ru alloys are preferred catalystswhen using reformed gases as the fuel. Pure Pt is a preferred catalystfor the cathode in PEMFCs. The anode and cathode may also, optionally,include additional components that enhance the fuel cell operation.These include, but are not limited to, an electronic conductor, forexample carbon, and an ionic conductor, for example a perfluorosulfonicacid based polymer or other appropriate ion exchange resin.Additionally, the electrodes are typically porous as well, to allow gasaccess to the catalyst present in the structure.

A fuel cell 73 can also be formed from the instant invention. As shownin FIG. 7, such PEM fuel cells 73 comprise the CCM 70 and may optionallyalso include gas diffusion layers 74 and 75 on the cathode 72 and anode71 sides, respectively. These GDM function to more efficiently dispersethe fuel and oxidant. The fuel cell may optionally comprise plates (notshown in FIG. 7) containing grooves or other means to more efficientlydistribute the gases to the gas diffusion layers. As is known in theart, the gas diffusion layers 74 and 75 may optionally comprise amacroporous diffusion layer as well as a microporous diffusion layer.Microporous diffusion layers known in the art include coatingscomprising carbon and optionally PTFE, as well as free standingmicroporous layers comprising carbon and ePTFE, for example CARBEL® MPgas diffusion media available from W. L. Gore & Associates. The fluidsused as fuel and oxidant may comprise either a gas or liquid. Gaseousfuel and oxidant are preferable, and a particularly preferable fuelcomprises hydrogen. A particularly preferable oxidant comprises oxygen.

The following test procedures were employed on samples which wereprepared in accordance with the teachings of the present invention.

Test Procedures Cell Hardware and Assembly

For all examples, standard hardware with a 23.04 cm² active area wasused for membrane electrode assembly (MEA) performance evaluation. Thishardware is henceforth referred to as “standard hardware” in the rest ofthis application. The standard hardware consisted of graphite blockswith triple channel serpentine flow fields on both the anode and cathodesides. The path length is 5 cm and the groove dimensions are 0.70 mmwide by 0.84 mm deep.

Two different cell assembly procedures were used. In the firstprocedure, designated as procedure No. 1, the gas diffusion media (GDM)used was a microporous layer of Carbel® MP 30Z placed on top of aCarbel® CL gas diffusion layer (GDM), both available from W. L. Gore &Associates, Elkton, Md. Cells were assembled with two 10 mil UNIVERSAL®ePTFE gaskets from W. L. Gore & Associates, having a square window of5.0 cm×5.0 cm, two 2.0 mil polyethylene naphthalate (PEN) films(available from Tekra Corp., Charlotte, N.C.) gaskets hereafter referredto as the spacer, and two 1.0 mil polyethylene naphthalate (PEN) filmshereafter referred to as the sub-gasket. The sub-gasket had an openwindow of 4.8×4.8 cm on both the anode and cathode sides, resulting in aMEA active area of 23.04 cm².

In the second procedure, designated as procedure No. 2, assemblymaterials were the same as procedure No. 1, with the exceptions that theGDM used was Carbel® CL GDM alone, and no spacers were incorporated.

All the cells were built using spring-washers on the tightened bolts tomaintain a fixed load on the cell during operation. They are referred toas spring-loaded cells. The assembly procedure for the cells was asfollows:

-   1. The 25 cm² triple serpentine channel design flow field (provided    by Fuel Cell Technologies, Inc, Albuquerque, N. Mex.) was placed on    a workbench;-   2. One piece of 10 mil ePTFE gasket with a 2.0 mil PEN spacer was    placed on anode side of the flow field;-   3. One set of the GDM was placed inside the gasket so that the    MP-30Z layer was facing up;-   4. The window-shaped sub-gasket of PEN sub-gasket sized so it    slightly overlapped the GDM on all sides was placed on top of the    GDM;-   5. The anode/membrane/cathode system was placed on top of the    sub-gasket with anode-side down;-   6. Steps (2) through (4) were repeated in reverse order to form the    cathode compartment. The gasket used on the cathode side was the    same as that used on the anode side.-   7. There are total of eight bolts used in each cell, all bolts had    spring washers, Belleville disc springs, purchased from MSC    Industrial Supply Co. (Cat#8777849). The bolts were then tightened    to a fixed distance that previously had been established to provide    a compressive pressure of 100-120 psi in the active area.    Compression pressure was measured by using Pressurex® Super Low Film    pressure paper from Sensor Products, Inc., East Hanover, N.J.

Fuel Cell Life Testing

Because the inventive membranes typically last a very long time(thousands of hours) under normal fuel cell operating conditions, twodifferent types of accelerated test protocols were developed toestablish membrane lifetimes. These protocols, identified as TestProtocol 1 and Test Protocol 2, are described more fully below.

Test Protocol 1

Materials to be tested were prepared as outlined below in the examples,and then assembled into a cell using the procedure outlined above. Thecell was connected to a test station, conditioned, and then the test wasstarted under test temperature and pressure as outlined below. Theassembled cells were tested in fuel cell test stations with GlobeTechgas units 3-1-5-INJ-PT-EWM (GlobeTech, Inc., Albuquerque, N. Mex.), andScribner load units 890B (Scribner Associates, Southern Pines, N.C.).The humidification bottles in these stations were replaced by bottlespurchased from Electrochem Corporation (Woburn, Mass.). The humidityduring testing was carefully controlled by maintaining the bottletemperatures, and by heating all inlet lines between the station and thecell to four degrees higher than the bottle temperatures to prevent anycondensation in the lines. In all cases the inlet and/or outlet relativehumidity of the anode and/or cathode was measured independently usingdew point probes from Vaisala (Vantaa, Finland) to ensure the inputhydrogen and air were humidified to desired relative humidity (RH) atthe testing temperatures.

The cells were first conditioned at a cell temperature 80° C. with 100%relative humidity inlet gases on both the anode and cathode. The outletgas pressure of both sides was controlled to be 15 psig. The gas appliedto the anode was laboratory grade hydrogen supplied at a flow rate of1.3 times greater than what is needed to maintain the rate of hydrogenconversion in the cell as to determined by the current in the cell(i.e., 1.3 times stoichiometry). Filtered, compressed and dried air wassupplied to the cathode humidification bottle at a flow rate of 2.0times stoichiometry.

The cells were conditioned for 4 hours. The conditioning processinvolved cycling the cell at 80° C. between a set potential of 600 mVfor 30 seconds, 300 mV for 30 seconds and 950 mV for 5 seconds for 4hours. Then a polarization curve was taken by controlling the appliedpotential beginning at 600 mV and then stepping the potential in 50 mVincrements downwards to 400 mV, then back upward to 900 mV in 50 mVincrements, recording the steady state current at every step. The opencircuit voltage was recorded between the potential steps of 600 mV and650 mV.

After the above procedure, the cells were set to the life-testconditions. This time was considered to be the start of the life test,i.e., time equal to zero for all life determinations. Specific testconditions in this protocol were (Table 2): cell temperature of 95° C.,50% RH for both hydrogen and air, with a stoichiometry of 1.3 and 2.0,respectively. Outlet pressure was 25 psig in all cases. The currentdensity of the cells in Protocol No. 1A and Protocol No. 1B wascontrolled to be 100, and 800 mA/cm², respectively.

TABLE 2 Operation Conditions for Accelerated Chemical Degradation TestsCell Gas Current Outlet Pressure Protocol Temp. Gas Type Inlet RH (%)Stoichiometry Density (anode/cathode) No. (° C.) (anode/cathode)(anode/cathode) (anode/cathode) (mA/cm²) (psig) 1A 95 H₂/Air 50/501.3/2.0 100 25/25 1B 95 H₂/Air 50/50 1.3/2.0 800 25/25

Test Protocol 2

In test Protocol 2, the materials were prepared as described below inthe examples, and assembled into cells as described above. The cellswere then conditioned, and subsequently tested using the procedureoutlined more fully below. Life of the membrane was determined using thephysical pin-hole test described below.

The test stations used for this protocol were fuel cell test stationswith Teledyne Medusa gas units Medusa RD-890B-1050/500125 (TeledyneEnergy Systems, Hunt Valley, Md.), and Scribner load units 890B. The gasunits were modified with additions of solenoid valves from Parkeroutside of the humidification bottles. These valves control directionsof gas flow so that the cells can be tested in wet and dry cycles.

The conditioning procedure used in this protocol was as follows: thecells were first conditioned at a cell temperature 70° C. with fullyhumidified (100% RH) inlet gases. The gas applied to the anode waslaboratory grade hydrogen supplied at a flow rate of the greater of 150cc/min or 1.2 times greater than what is needed to maintain the rate ofhydrogen conversion in the cell as determined by the current in the cell(i.e., 1.2 times stoichiometry). Filtered, compressed and dried air wassupplied to the cathode at a flow rate of the greater of 650 cc/min ortwo times stoichiometry. Then, the cells were continuously cycled at 70°C. by fixing a set potential of 600 mV for 45 seconds, followed by opencircuit voltage (OCV) for 30 seconds, 300 mV for 60 seconds, and finallyOCV for 30 seconds. This cycling was repeated continuously for 10 hours.Then a polarization curve was taken by controlling the applied potentialbeginning at 600 mV for 8 minutes and then stepping through thefollowing potentials and times intervals: 500 mV for 8 minutes, 400 mVfor 8 minutes, 450 mV for 8 minutes, 550 mV for 8 minutes, 650 mV for 8minutes, 750 mV for 8 minutes, 850 mV for 6 minutes, 900 mV for 4minutes, 800 mV for 6 minutes, 700 mV for 8 minutes, 600 mV for 8minutes, recording the steady state current at every step. Then thefollowing current densities were applied in steps: 100 mA/cm² for 3minutes, 500 mA/cm² for 3 minutes, 800 mA/cm² for 3 minutes, and finallythe cell was left at open circuit potential for 2 minutes, recording thesteady state potential at every step.

After the above procedure, the cells remain at 700 mV for between 0 and24 hours. Then the cell was pressured to 25 psig. The cells were furtherconditioned at a cell temperature of 80° C. with dry relative humidityinlet gases on both the anode and cathode. The hydrogen gas applied tothe anode was at a utilization of 0.83 with a minimum flow rate of 50cm³/min. Filtered, compressed and dried air was supplied to the cathodeat a flow rate of the greater of 100 cm³/min or 4.0 times stoichiometry.The current was set to 200 mA/cm² for 30 minutes and the potential wasrecorded. Then the cell was changed to 100% RH inlet gases for 90seconds. An open circuit voltage decay measurement was then initiated bystopping the gas flow on the cathode and removing the load. Theresulting voltage was measured every 3 seconds for 1 minute as theoxygen on the cathode is consumed by hydrogen crossing over from theanode to cathode. This is a measure of membrane health at beginning oflife, under pressure. The flow on the anode was then set to 150 cc/minand 650 cc/min on the cathode for 10 seconds under no load. Then theload is applied at 800 mV for 20 seconds. Finally the anode flow was setto the greater of 50 cc/min or 1.2 times stoichiometry and cathode flowis set to the greater of 100 cc/min or four times stoichiometry. Thecurrent was set to 200 mA/cm² for 30 seconds and the potential isrecorded. This ends the initial conditioning.

After initial conditioning and diagnostics, the MEA was tested under thefollowing test conditions. The cell temperature remained at 80° C. Thecell was pressurized on the anode with hydrogen and on the cathode withair to 25 psig. The hydrogen flow rate on the anode was at 1.2 timesstoichiometry with a minimum flow rate of 50 cm³/min. Air was suppliedto the cathode at a flow rate of the greater of 100 cm³/min or 4.0 timesstoichiometry. The current was set to 200 mA/cm² and potential wasrecorded. The inlet gas was cycled from by pass of the humidificationbottles to flow through the humidification bottles. This cycling iscontrolled by the solenoid valves that switch every 45 seconds. Theresult was an inlet humidification that rises and falls every 45seconds. The inlet gases reach the following maximum and minimumhumidification:

Anode wet condition 61° C. dew point=44% RH

Anode dry condition 31° C. dew point=10% RH

Cathode wet condition 75° C. dew point=80% RH

Cathode dry condition 14-20° C. dew point=3-5% RH

During the test, the open circuit voltage (OCV) decay was measured twotimes every hour. The first measurement was done after a 45 second wetcycle, and the second 30 minutes later after a 45 second dry cycle.These measurements were made under pressure and automatically by thetest station, as described above except that the air flow to the cathodewas shut off for 3 minutes, instead of 1 minute.

Chemical Degradation Rate:

For all the tests the amount of fluoride ions released into the productwater was monitored as a means to evaluate chemical degradation rate.This is a well-known technique to establish degradation of fuel cellmaterials that contain perfluorosulfonic acid ionomers. Product water offuel cell reactions was collected at the exhaust ports throughout thetests using PTFE coated stainless steel containers. The collected waterwas then concentrated about 20 fold (for example, 2000 ml to 100 ml) inPTFE beakers heated on hot plates. Before concentration, 1 ml of 1M KOHwas added into the beaker to prevent evaporation of HF. Fluorideconcentration in the concentrated water was determined using anF-specific electrode (ORION® 960900 by Orion Research, Inc.). Fluoriderelease rate in terms grams F/cm²-hr) was then calculated.

Membrane Life Measurement

The life of the membrane was established by determining the presence offlaws in the membrane that allow hydrogen to cross through it. In thisapplication, this so-called hydrogen cross-over measurement was madeusing a flow test that measures hydrogen flow across the membrane.Because this test is somewhat tedious, and may itself weaken themembrane, it was only performed when there was an indication that theintegrity of the membrane was questionable. The membrane integrity wasthus first evaluated during testing using an OCV decay measurementperformed at ambient pressures. In Test Protocol 1, this measurement wascarried out while the cell remained as close as possible to the actuallife test condition. In Test Protocol 2 this measurement was performedunder 100% RH conditions. This ambient OCV decay test was performedperiodically as indicated by the performance of the cell. Typically, itwas performed less frequently near the beginning of cell life (e.g.,once a week), and more frequently the longer the cell operated (e.g., asoften as once per day toward the end of life). Details of themeasurement were as follows:

-   1. The cell was set at 0.6V, anode and cathode minimum flow rate to    be 800 cc/min. and 0 cc/min, respectively;-   2. The outlet pressure of anode and cathode side was reduced to 2.0    and 0 psig, respectively;-   3. The cell was then taken off load while remaining at the test    temperature; meanwhile, outlet flow of the cathode side was shut off    by a valve;-   4. The OCV value was recorded every second for 180 seconds;-   5. The decay in the OCV during this measurement was examined. If    this decay was significantly higher than previously observed, e.g.,    when the open circuit voltage value decayed to less than 250 mV in    less than 30 seconds, a physical flow check was initiated to    determine if the membrane had failed;-   6. If the decay was close to that of the previous measurement, the    life testing was resumed. When a physical flow check was indicated,    it was performed as follows:-   7. The cell was taken off load, and set at open circuit condition    while maintaining the cell temperature and RH conditions at the    inlets. The gas pressure of the cell was then reduced to ambient    pressure on both anode and cathode sides.-   8. The gas inlet on the cathode was disconnected from its gas supply    and capped tightly. The cathode outlet was then connected to a flow    meter (Agilent® Optiflow 420 by Shimadzu Scientific Instruments,    Inc., Columbia, Md.). The anode inlet remained connected to the H₂    supply and anode outlet remained connected to the vent.-   9. The anode gas flow was increased to 800 cc/min, and the anode    outlet pressure was increased to 2 psi above ambient pressure.-   10. In Test Protocol 2, the H₂ gas is supplied at 0% RH for 30    minutes.-   11. The amount of gas flow through the cathode outlet was measured    using the flow meter.-   12. A failure criterion of 2.5 cc/min was established, so that when    the measured gas flow of H₂ was greater than this value, the    membrane was identified as having failed.-   13. If the criterion for failure was met the test was stopped, and    the membrane life was recorded as the number of hours the cell had    been under actual test conditions when it failed the physical flow    check (>2.5 cc/min). If the criterion for failure was not met, the    cell was returned to testing.

Mechanical Property Measurement

Certain membranes were subjected to mechanical testing at roomconditions of 21° C. and 60% RH. A dynamic mechanical analyzer (DMA) (TAInstruments, Wilmington, Del.) mode RSA3 was used. Each membrane typetested in machine as well as transverse directions. The membrane was diecut to a rectangular shape with a width of 4.8 mm, and a length of 50mm. The grip gap was set to be 15 mm, and the membrane sample was pulledat the rate of 0.5 mm/s until failure. Tension force during the sampleelongation was recorded, and the maximum value before sample failure wasregarded as the failure force. During data analysis, values of forcewere plotted against elongation. The slope of the linear portion of thecurve, specifically, from 0 to 0.04 elongation portion of the curve wascalculated as the stiffness. Cross sectional area for each sample wascalculated using sample width, i.e. 4.8 mm, times sample thickness.Values of failure force and stiffness were divided by sample's crosssection area to obtain strength and modulus values, respectively. Thelesser values of the mechanical properties from the transverse andmachine direction are those reported here as Failure Force, Stiffness,Strength and Modulus.

Platinum Loading Measurement

To confirm that the platinum used to prepare the inventive solid polymerelectrolytes had not been lost in processing, the amount of platinum inthe membranes was measured using a bench-top x-ray fluorescence unit(XRF from SPECTRO TITAN, Kleve, Germany) pre-calibrated to display Ptcontent in units of mg Pt per cm² surface area. Three separatemeasurements of the concentration of sections of the as-prepared solidpolymer electrolytes were taken by placing the as-prepared inventivesolid polymer electrolyte in the unit and recording the displayedvalues. The values reported in the examples below are the average valuesof the three measurements taken for each material. In all cases, themeasured amounts were equal to the expected amounts within experimentalerror of the measurement.

Transmission Electron Microscopy and Interparticle Spacing Measurement

In order to observe distribution of supported catalysts inside themembrane, transmission electron microscopy (TEM) was performed on crosssections of selected inventive solid polymer electrolytes. A section ofthe solid polymer electrolyte was embedded in Spurr® epoxy resin andcured at 60° C. for eight hours. The embedded sample was first trimmedwith a razor blade and then thin sectioned at room temperature using aDiatome diamond knife on a Leica Ultracut UCT ultramicrotome. Themicrotome was set to cut 75 nm thick sections which were collected on300 mesh copper grids. TEM was performed using a JEM 2010 Field EmissionTEM, at 200 KV at various magnifications. The interparticle spacingbetween support particles was determined as follows: a micrographrepresentative of the observed microstructure was obtained at amagnification where a large number of the plurality of support particlescould be seen, at least 20, and preferably at least 50. The distancebetween 15 different pairs of surrounding neighbors of the plurality ofsupport particles chosen at random was measured. The interparticlespacing was calculated as the average of the 15 measurements. Todetermine the interparticle spacing between catalyst particles, thefollowing procedure was used: a micrograph representative of thecatalyst particles on the support particle was obtained at amagnification where at least one support particle could be observed, anda plurality of catalyst particles on the support could be seen, at least4, and preferably 6 or more. The distance between 6 to 10 differentpairs of the plurality of support particles chosen at random wasmeasured, and the interparticle spacing was calculated as the average ofthe measurements.

Without intending to limit the scope of the present invention, the solidpolymer electrolytes and method of production of the present inventionmay be better understood by referring to the following examples

EXAMPLES

In the examples below, three different ion exchange materials were usedto prepare solid polymer electrolytes. The first material, identifiedherein as Type 1, was prepared according to the teachings of Wu, et. alin U.S. Patent Application 20030146148, Example 5 except the reactantswere adjusted to produce a product with equivalent weight of about 920.

This polymer had a melt flow index (MFI) that was typically 6±2 g/10 minwith a range between 2 and 12. The MFI was measured by placing a 2160gram weight onto a piston on a 0.8 cm long die with a 0.20955 cmorifice, into which 3-5 grams of as-produced polymer had been placed.Three separate measurements of the weight of polymer that flowed throughthe orifice in 10 minutes at 150° C. was recorded. The MFI in g/10 minwas calculated as the average weight from the three measurements times3.333. To make the ion exchange material more stable, this product wastreated with 500 kPa fluorine gas at 60° C. in one five-hour cycle andthree four-hour cycles, each one separated by an evacuation step,essentially according to the teachings in GB 1,210,794. The polymer wassubsequently extruded, pelletized and acidified using proceduresstandard in the art. Then it was made into a dispersion by forming asolution of 20%-30% of the acid form of the Type 1 polymer, 10-20%deionized water, and balance alcohol in a glass-lined pressure vessel.The vessel was sealed, and the temperature was raised to 140° C. at arate slow enough to maintain the pressure at less than 125 psi. It washeld at 140° C. and about 125 psi for 2.5 hours. Then, a final solutionwas obtained by adding sufficient water to produce a solution consistingof approximately 20% solids, 20% water and 60% alcohol.

A second ion exchange material, Type 2, was prepared in the same way asType 1, but the fluorine gas treatment of the ion exchange polymer waseffected at 135° C. in 500 kPa of 20% fluorine/80% nitrogen for two 4hour periods and two 6 hour periods. The acid form of this polymer wasformed into a dispersion as described above for the Type 1 polymerexcept the temperature and pressure during the solution preparationprocess was 160° C. and 210 psi. The MFI of this polymer was typically4.4 g/10 min with a range between 2 and 12.

The final polymer, Type 3, was prepared as described for Type 1 but ithad an MFI of about 0.9 g/10 min. It was treated with fluorine gas inthe same fashion as Type 2. The polymer was then made into a dispersionby forming a solution of 10% of the acid form of the polymer, and thebalance ethanol in a glass-lined pressure vessel. The vessel was sealed,and the temperature was raised to 140° C. at a rate slow enough tomaintain the pressure at less than 125 psi, and held at 140° C./125 psifor 2.5 hours. Then, a final solution was obtained by adding a weight ofwater approximately equal to the polymer weight and then concentratingthe solution by evaporating the solvent at room temperature. The finalsolution then consisted of approximately 20% solids, 20% water and 60%alcohol.

Example 1

In Example 1, a solid polymer electrolyte membrane was prepared asfollows: A concentrated catalyst ink consisting of platinum on a carbonsupport (type V11-D50 catalyst, Englehard Corporation, Iselin, N.J.) ata 1:1 weight ratio (35% water by weight), Type 1 ion exchange material,and normal propanol in the following approximate ratios, respectively,8.54%, 4.27%, and 87.19% was prepared. This was accomplished as follows.A slurry was prepared by mixing a portion of the n-propanol with thecatalyst powder in an 30 liter glass reactor (H. S. Martin, Inc.,Vineland, N.J.) after evacuating it, and refilling with nitrogen.Subsequently, the slurry was pumped into a 50 liter vessel whereagitation was supplied for 20 minutes by a rotor/stator agitator (ModelAX200 by Silverson Machines Inc., Longmeadow, Mass.) while the solutionwas recirculated through a ISG motionless static mixer (Charles Ross &Sons, Hauppauge, N.Y.). To this slurry, the ionomer was addedcontinuously over about 45 minutes. The solution of ionomer, solvent andcatalyst was further mixed in the same container for an additional 30minutes. Then, the solution was recirculated through a Model M-700Microfluidizer (Microfluidics, Newton, Mass.) at 10,000 psig for 45minutes. Finally, the solution was further mixed using the Silversonmixer with recirculation for an additional 20 minutes. The finalconcentrated ink solution was pumped into a holding tank, the systemflushed with rinse solvent that was subsequently also pumped into theholding tank. The solution was stirred continuously for a five dayperiod with a low shear propeller agitation system and stored in aplastic container for a period of time ranging from a few days to a fewweeks. Immediately before use, this ink was passed through aMicrofluidizer at 19,000 psig three times. It was then stirred with amagnetic stir bar until use, generally within about 30 minutes.

An inventive solid polymer electrolyte membrane was prepared as follows.First, an expanded polytetrafluoroethylene (ePTFE) membrane was preparedwith mass per area of 7.0 g/m², thickness of 20 microns, and porosity ofat least 85%, and a longitudinal matrix tensile strength of about 67MPa, and a transverse matrix tensile strength of about 76 MPa using theteachings of U.S. No. 3,953,566 to Gore. The ink prepared above was thendiluted with Type 1 ion exchange material to give a concentration of0.8% platinum based on weight percent of dry ionomer solids. This ionexchange material solution was coated on a polyethylene naphthalate(PEN) film stretched onto a glass plate using a drawdown blade on whichthe coating gap can be adjusted between 1 and 10 mil. For this firstcoating, the gap was adjusted to 0.0038 inches (0.00965 cm). The ePTFEmembrane was then stretched over the wet coating and allowed toinfiltrate. After infiltration, it was dried for 20-60 s with a hairdrier. Then, a second coating of the same ion exchange material solutionwas applied with a 0.0019 inch (0.00483 cm) gap set on the draw bar. Thesecond coating was then also dried with a hair drier for 20-60 s. Thismembrane was placed in a 160° C. air furnace for three minutes and thenremoved to cool. The membrane was then removed from the PEN backer beingcareful not to stretch it severely. The measured platinum loading ofthis membrane was 0.015 mg/cm², and the final thickness of the solidpolymer electrolyte was 18 microns.

The mechanical properties of a section of this solid polymer electrolytewere tested using the procedures described above. The results for theFailure Force, Stiffness, Strength and Modulus are shown in Table 6.

Another section of the completed solid polymer electrolyte was placedbetween two PRIMEA® 5510 electrodes (available from Japan Gore-Tex,Inc., Tokyo, Japan) with 0.4 mg Pt/cm² loading in the each electrode.This sandwich was placed between platens of a hydraulic press (PHI Inc,Model B-257H-3-MI-X20) with heated platens. The top platen was heated to180 degrees C. A piece of 0.25″ thick GR® sheet (available from W. L.Gore & Associates, Elkton, Md.) was placed between each platen and theelectrode. 15 tons of pressure was applied for 3 minutes to the systemto bond the electrodes to the membrane. This MEA was assembled into afuel cell as described above, and tested under Test Condition 1A. TheLifetime and Fluoride Release Rate were measured, and results are shownin Table 2.

Example 2

To illustrate the importance of the mechanical properties of the solidpolymer electrolytes to the inventive materials herein, a material wasmade that is otherwise identical to an embodiment of the inventivematerials, but has weaker mechanical properties. It thus has lowfluoride release rates indicative of a chemically stable membrane, butits life is not as long as Example 1 because the polymer electrolytemembrane is not as strong as that formed in Example 1. The solid polymerelectrolyte of this example was prepared as follows. First, an ePTFEmembrane was prepared according to the teachings of Gore in '566 with anaverage mass per area of about 3.3 g/m², a thickness of about 7.8microns, an average ball burst strength of about 1.18 lbs, and anaverage Frazier number of about 42 ft3/min/ft2 at 0.5 inches of water.The ball burst is a standard test (see for example, U.S. Pat. No.5,814,405 to Branca, et. al.) performed on porous membranes thatmeasures the relative strength of a sample of membrane by determiningthe maximum load at break. A single layer of membrane is challenged with1 inch diameter ball while being clamped and restrained in a ring of 3inch inside diameter. The membrane is placed taut in the ring andpressure applied against it by the steel ball of the ball burst probe.Maximum load is recorded as “Ball Burst” in pounds.

An ink was prepared as described in Example 1 using ion exchangematerial Type 3 to give a concentration of 0.8% platinum based on weightpercent of dry ionomer solids. The as-prepared ink was passed throughthe Microfluidizer three consecutive times with a pressure setting of19,000 psi. Then the solid polymer electrolyte was prepared as follows:for the first coating, a #44 Meyer Bar was used to coat onto a PEN filmstretched tight over a glass plate using the prepared ink. The ePTFEmembrane was then stretched over the wet coating and allowed toinfiltrate. After infiltration, it was dried for 20-60 s with a hairdrier. Then, a second coating using the ink solution prepared above wasapplied with a #22 Meyer Bar. The second coating was then also driedwith a hair drier for 20-60 s. This membrane was placed in a 160° C. airfurnace for three minutes and then removed to cool. The membrane wasthen removed from the backer in room temperature deionized water. Themeasured platinum loading of this membrane was 0.017 mg/cm², and itsfinal thickness was 20 microns.

A section of the same material was made into an MEA as described above,and assembled in a fuel cell. Testing using Test Protocol 1A showedLifetime of 78 h and Fluoride Release Rate of 4.5×10⁻⁸. This is to becompared, for example, with the Lifetimes from Example 1 of nearly anorder of magnitude higher (718 h).

The mechanical properties were measured on a different section of thissolid polymer electrolyte. The results (Table 6) showed that it wasweaker than Examples 1, 5 and 7, having for example, a Failure Force of106 g. This illustrates that although all the inventive materials havelow fluoride release rates, the combination of a layer containing acatalyst of a supporting particle, and a strong solid polymerelectrolyte are essential for long Lifetimes.

Example 3

To confirm the importance of the mechanical properties together with thecomposite layer comprising a catalyst on a supporting particle, anothersolid polymer electrolyte was prepared that had a composite layer ofcatalyst on supporting particle but had low mechanical properties. Thiswas done as follows. A Type 1 ion exchange material with 0.8% Pt to dryionomer weight was prepared using the procedures of Example 1. Thedrawdown bar was set to 0.025 cm (0.010 inches) and only one coating wasdone directly onto a glass substrate (no polymer film). No ePTFE wasused. After drying, the solid polymer electrolyte was removed in roomtemperature water. The resulting solid polymer electrolyte was 20 to 26microns thick.

The mechanical properties measured on a separate piece of this samematerial show that it is significantly weaker than those measured on thematerial of Ex. 1 (Table 6)

A different section of this solid polymer electrolyte was prepared intoan MEA using the procedures above, assembled into a fuel cell, andtested using Test Protocol 1A. The results show that the fluoriderelease rates are very low.

Comparative Example 1

A PRIMEA® series 5700 MEA with 0.4 mg Pt/cm² loading (W. L. Gore &Associates, Elkton, Md.) in each electrode was assembled into a fuelcell as described above and tested in Test Condition 1A. This MEA isreinforced with ePTFE and is the latest commercial offering (as of thedate of filing) from W. L. Gore & Associates, so provides an indicationof state-of-the-art performance for durable, composite membranes. Thereis no catalyst present in the solid polymer electrolyte of this catalystcoated membrane. The results shown in Table 2 indicate that the MEA inExample 1 using the inventive solid polymer electrolyte has nearly threetimes the life of the MEA of this Comparative Example, and has 1-2orders of magnitude higher fluoride release rate than the inventiveExamples 1-3.

TABLE 2 Fluoride Lifetime Release Rate Example No. (hr) (g/hr · cm²) Ex.1 718 1.10E−07 Ex. 2. 78 4.50E−08 Ex. 3 90 7.10E−08 Comp. Ex. 1 2431.30E−06

Example 4

Another section of the solid polymer electrolyte that was prepared inExample 1 was used to prepare an MEA in the same fashion as Example 1.It was assembled into a fuel cell using the procedures described above,and tested using Test Protocol 1B. The Lifetime and Fluoride ReleaseRate results are shown in Table 3.

Comparative Example 2

In order to compare the inventive solid polymer electrolyte materials tothose prepared previously in the prior art, a solid polymer electrolytewas prepared using a procedure similar to that used in U.S. Pat. No.5,472,799 to Watanabe et. al. Specifically, a dispersion of unsupportedplatinum particles was formed in a solid polymer electrolyte by thefollowing procedure:

-   -   1) 0.219 grams of hydrogen hexachloroplatinate (IV) hydrate salt        (H₂Cl₆Pt.H₂O) (available from Sigma-Aldrich, St. Louis, Mo.) was        dissolved in 10 grams of FLEMION® dispersion with equivalent        weight of 950 (Asahi Glass Co. Ltd, Chemicals, Tokyo, JAPAN) in        a 9% solid ionomer water/alcohol solution using a magnetic stir        bar and a stir plate;    -   2) 25 cm³ of water solution of sodium boron hydride (NaBH₄ from        Sigma-Aldrich, St. Louis, Mo.) with concentration of 0.05 M was        then prepared;    -   3) The NaBH₄ solution was titrated into the H₂Cl₆Pt containing        ionomer solution slowly. During the titration, platinum ions        (Pt⁴⁺) are reduced to colloidal platinum metal (Pt) particles        through the reduction effect of NaBH₄. As this reaction        proceeded further, the ion exchange solution turned to a dark        color due to increasing amount of colloidal Pt particles. The        relative concentration of Pt⁴⁺ and BH₄ ⁻ was monitored by        measuring the electrochemical potential difference between a Pt        wire working electrode and a Hg/HgSO⁴ reference electrode        emerged in the ion exchange solution. The end-point of the        titration was marked by a sudden drop of the electrochemical        potential;    -   4) After reaching the end-point, the ionomer mixture was poured        out into a shallow glass dish and dried at room temperature        under dry nitrogen flow;    -   5) The dried ionomer mixture was washed using 0.05 M, high        purity sulfuric acid (H₂SO₄) solution to eliminate ions such as        chlorine (Cr) and sodium (Na⁺) ions;    -   6) After washing with acid three times, the ionomer was washed        with de-ionized water three times;    -   7) The cleaned ionomer was dissolved into water/alcohol at room        temperature using a magnetic stir bar on a stir plate to obtain        a solution consisting of approximately 20% solids, 20% water and        60% alcohol. Within the ionomer solids, pure Pt colloidal        particles accounted for approximately 1% by weight.    -   8) A solid polymer electrolyte was then prepared from this        solution using a process similar to that used in Example 4.        Specifically a roll of expanded polytetrafluoroethylene (ePTFE)        membrane with mass per area of 7.0 g/m², a thickness of 20        microns, a porosity of at least 85%, a longitudinal matrix        tensile strength of about 67 MPa, and a transverse matrix        tensile strength of about 76 MPa, was prepared using the        teachings of U.S. Pat. No. 3,953,566 to Gore. Then, the solution        of prepared in (7) was coated onto a film paid off of a roll of        PEN film at 3 feet/min using a #44 Meyer bars. The ePTFE was        then applied to the wet solution, and passed through a 3 ft oven        held at 150° C. in air. A second coating of the solution was        made onto this material by running it through the same line at 3        feet/min using the same Meyer Bar. The final solid polymer        electrolyte was 25 microns thick, and had a platinum loading in        the solid polymer electrolyte of 0.016 mg/cm².

A TEM micrograph of this membrane shows the presence of platinumparticles 80 in FIG. 8.

This solid polymer electrolyte was used to prepare an MEA and thenplaced into a fuel cell using the procedures described above. It wasthen tested using Test Protocol 1B. The results (Table 3) indicate thatthe lifetimes are shorter, and the fluoride release rates are higherthan the inventive membranes (Ex. 4).

Comparative Example 3

A PRIMEA® series 5700 MEA with 0.4 mg Pt/cm² loading (W. L. Gore &Associates, Elkton, Md.) in each electrode was assembled into a fuelcell as described above and tested in Test Condition 1B. This MEA isreinforced with ePTFE and is the latest commercial offering (as of thedate of filing) from W. L. Gore & Associates, so provides an indicationof state-of-the-art performance for durable, composite membranes. Thereis no catalyst present in the solid polymer electrolyte of this catalystcoated membrane. The results shown in Table 3 indicate that the MEA inExample 4 using the inventive solid polymer electrolyte has nearly twicethe life of the MEA of this Comparative Example, and over seven timeslower fluoride release rate.

TABLE 3 Fluoride Lifetime Release Rate Example No. (hr) (g/hr · cm²) Ex.4 1365 4.30E−08 Comp. Ex. 2 527  2.3E−07 Comp. Ex. 3 700 1.90E−07

Example 5

In this example an inventive solid polymer electrolyte was prepared withplatinum in a layer on only one side of the final solid polymerelectrolyte. This was done as follows. First, an expandedpolytetrafluoroethylene (ePTFE) to membrane was prepared using theteachings of Hobson et. al. in U.S. Pat. No. 6,613,203, incorporatedherein in its entirety. A membrane similar to the Type 2 ePTFE in Hobsonwas prepared except the processing parameters were adjusted so the massper area was about 7.5 g/m², the thickness was 25 microns, thelongitudinal matrix tensile strength was about 267 MPa (38,725 psi), thetransverse matrix tensile strength was about 282 MPa (40,900 psi), theGurley number was about 8.5 seconds, and the aspect ratio was about 29.An ink prepared as described in Example 1 was mixed with ion exchangematerial Type 3 to give a concentration of 2.4% platinum based on weightpercent of dry ionomer solids. This solution was passed through theMicrofluidizer three consecutive times with a pressure setting of 19,000psi. Then the solid polymer electrolyte was prepared as follows: for thefirst coating, a #44 Meyer Bar was used to coat onto a PEN filmstretched tight over a glass plate. Pure Type 3 (with no platinum in it)was used for this first coating. The ePTFE membrane was then stretchedover the wet coating and allowed to infiltrate. After infiltration, itwas dried for 20-60 s with a hair drier. Then, a second coating usingthe ink solution prepared above was applied with a #22 Meyer Bar. Thesecond coating was then also dried with a hair drier for 20-60 s. Thismembrane was placed in a 160° C. air furnace for three minutes and thenremoved to cool. The membrane was then removed from the backer in roomtemperature deionized water. The measured platinum loading of thismembrane was 0.022 mg/cm², and its final thickness was 18 microns. Thismaterials was tested in a Gurley Densometer Model 4110 (Gurley PrecisionInstruments, Troy, N.Y.) and found to have a Gurley number greater than10,000 s.

In this example, an MEA was prepared with a section of the solid polymerelectrolyte using the procedure described in Example 1. This MEA wasplaced in a fuel cell using the procedures described above, so that theside with the layer containing carbon particles supporting platinum wasfacing the anode compartment. It was then tested using Test Protocol 1A.The Lifetime and Fluoride Release Rate results are shown in Table 4. Themechanical properties of a separate section of the solid polymerelectrolyte were also obtained, with the results also shown in Table 6.

Example 6

A different section of the solid polymer electrolyte prepared in Example5 was made into an MEA as described in Example 5 and tested in a fuelcell using Test Protocol 1A. In this Example, though, the side with thelayer containing carbon particles supporting platinum was facing thecathode compartment. The lifetime and fluoride release rate results areshown in Table 4.

Comparative Example 4

In order to obtain an indication of the improvement in properties of theinventive materials of Example 5 and 6, a material made with the samereinforcement and same ionomer used in Examples 5 and 6 was prepared. Asolid polymer electrolyte was prepared using the same methods describedabove, except only pure Type 3 ionomer was used so that no catalystsupported on a carbon layer was present. The results from testing inTest protocol 1A (Table 4) surprisingly show that the lifetime of theinventive materials was about two (Ex. 5) to over seven (Ex 6) timeshigher than this Comparative Example. The fluoride release rates werehalf (Ex. 5) to more than ten times (Ex. 6) lower than those observed inthis Comparative Example.

TABLE 4 Fluoride Lifetime Release Rate Example No. (hr) (g/hr · cm²) Ex.5 523 3.90E−07 Ex. 6 1677 3.40E−08 Comp. Ex. 4 283 8.60E−07

Example 7

In order to show that a microporous reinforcement is not required forthe inventive materials to achieve improved lifetimes, a NAFION® N101membrane was purchased from Ion Power, Inc. (Bear, Del.). Unlike someNAFIONO membranes, this material is processed in a way to make themembrane relatively strong. A layer of ion exchange materials comprisingcarbon supporting platinum catalyst was then laminated onto thismembrane to prepare an inventive solid polymer electrolyte. Theprocedure was as follows: first, a solid polymer electrolyte of ionexchange material Type 3 containing ink with a platinum concentration of2.4% was cast onto a fluoropolymer treated polyethylene terepthalate(PET) film using a #22 meyer bar. This membrane was dried at 80° C. for5 minute and removed from the PET film at room temperature in air. Ithad a thickness of five microns. This layer was then laminated to theN101 membrane at 180° C. for 1 minute. The final membrane had a measuredplatinum content of 0.019 mg/cm² and a thickness of 30 microns. Themechanical properties of a section of this membrane were tested, and theresults are shown in Table 6.

An MEA was prepared with this membrane and it was assembled into a fuelcell with the layer of carbon supporting platinum catalyst facing thecathode as described above. The results from testing in Test Protocol 1Aare shown in Table 5.

Comparative Example 5

A solid polymer electrolyte was prepared using the same proceduresoutlined in Example 7, except the cast Type 3 membrane had no ink in it,i.e., it was pure Type 3 ion exchange material with no platinumsupported on carbon in it. This cast membrane had a thickness of 5microns after drying. After lamination to the N101 membrane, theresulting membrane had a thickness of 30 microns. This solid polymerelectrolyte was tested using Test Protocal 1A and the same procedures asExample 7. The results (Table 5) show that the inventive solid polymerexchange material, Example 7, has nearly twice the lifetime, and morethan three times lower fluoride release rate than Comparative Example 5.

TABLE 5 Fluoride Lifetime Release Rate Example No. (hr) (g/hr · cm²) Ex.7 138 1.00E−06 Comp. Ex. 5 74 3.40E−06

TABLE 6 Failure Force Stiffness Strength Modulus Example No. (g) (g)(g/cm²) (g/cm²) Ex. 1 237 1911 2.63E+05 2.12E+06 Ex. 3 106 1547 1.18E+051.72E+06 Ex. 5 409 3310 4.54E+05 3.68E+06 Ex. 7 181 2724 1.21E+051.83E+06 Comp. Ex. 2 110 1772 9.15E+04 1.48E+06

Example 8

In order to demonstrate the utility of the inventive solid polymerelectrolytes under conditions that might occur in real applications, asample was prepared and tested in Test Protocol 2. The sample wasprepared with platinum supported on carbon in layers on two sides usingthe general procedures of Example 1. Here, though, the expandedpolytetrafluoroethylene (ePTFE) membrane was prepared using theteachings of Hobson et. al. in U.S. Pat. No. 6,613,203. A membranesimilar to the Type 2 ePTFE in Hobson was prepared except the processingparameters were adjusted so the mass per area was about 7.5 g/m², thethickness was 25 microns, the longitudinal matrix tensile strength wasabout 267 MPa (38,725 psi), the transverse matrix tensile strength wasabout 282 MPa (40,900 psi), the Gurley number was between 10 and 12seconds, and the aspect ration was about 29. The ink was prepared asdescribed in Example 1 using ion exchange material Type 1 to give aconcentration of 0.8% platinum based on weight percent of dry ionomersolids. In this example, the ink was used for both coating stepsdescribed in Example 1. In the first coating step the drawdown bar wasset to 0.0965 cm (0.038 inches), while in the second, it was set to0.0483 cm (0.019 inches). After drying the second coating with a hairdryer, the membrane was placed in a 160° C. air furnace for threeminutes and then removed to cool. The membrane was then removed from thePEN film in room temperature deionized water. The measured platinumloading of this membrane was 0.015 mg/cm², and its final thickness wasbetween 19 and 21 microns.

An MEA was prepared with a section of the solid polymer electrolyteusing the procedure described in Example 1. This MEA was placed in afuel cell using the procedures described above and tested using TestProtocol 2. The lifetime and fluoride release rate results are shown inTable 7.

In order to observe distribution of supported catalysts inside themembrane, transmission electron microscopy was performed on crosssections of the ion exchange membrane used in this example. A section ofthe solid polymer electrolyte of this example was embedded in Spurr®epoxy resin and cured at 60° C. for eight hours. The embedded sample wasfirst trimmed with a razor blade and then thin sectioned at roomtemperature using a Diatome diamond knife on a Leica Ultracut UCTultramicrotome. The microtome was set to cut 75 nm thick sections whichwere collected on 300 mesh copper grids. Transmission ElectronMicroscopy was performed using a JEM 2010 Field Emission TEM, at 200 KVat various magnifications. The results indicated that there was aplurality of very fine Pt/C particles of size less than 75 nm (FIG. 9).The interparticle spacing between these Pt/C particles was measuredbetween 15 different pairs of particles and found to be about 115 nm onaverage.

The presence of Pt and C in these particles was confirmed at highermagnifications (FIG. 10) both by contrast, and the presence of latticeimages consistent with C and Pt, 100 and 101, respectively in FIG. 9. Asan aid to the eye, the dotted line in FIG. 10 has been added to show theapproximate extent of a carbon particle supporting platinum. Theinterparticle spacing between these Pt particles was measured between 10different pairs of Pt particles and found to be about 10 nm on average.

Examples 9-10

An additional solid polymer electrolyte was prepared using the sameprocedure as Example 5 except that the final step of passing the dilutedink solution through the Microfluidizer was omitted. The measuredplatinum loading of this SPE was 0.016 mg/cm² and its final thicknesswas 15-18 microns. Two MEAs were prepared with sections of the solidpolymer electrolyte using the procedure described in Example 1. TheseMEAs were placed in a fuel cell using the procedures described above andtested using Test Protocol 2. The lifetime and fluoride release rateresults are shown in Table 7.

Example 11

To confirm the surprisingly low fluoride release rates of the inventivematerials, a different section of the solid polymer electrolyte preparedin Example 2 was made into an MEA using the procedures above, assembledinto a fuel cell, and tested using Test Protocol 2. The fluoride releaserate was very low, comparable to that observed in Example 2, which wastested in a different test protocol.

Example 12

To further confirm the surprisingly low fluoride release rate of theinventive materials, a different section of the solid polymerelectrolyte prepared in Example 3 was made into an MEA using theprocedures above, assembled into a fuel cell, and tested using TestProtocol 2. The fluoride release rate was again very low, comparable tothat observed in Example 3, which was tested under a different testprotocol.

Comparative Example 6

A sample to compare to Example 8 was prepared in this ComparativeExample. The preparation procedure was similar to that used in Example 8except that no ink was used in the preparation of the solution so therewas no platinum supported on carbon in the final SPE. A #28 meyer barwas used for the first coating (instead of the drawdown bar), a #22meyer bar was used for the second coating (instead of the drawdown bar),and the heat treatment after drying the second coating took place at150° C. for 1 minute. After removing the final solid polymer electrolytefrom the PEN film at room temperature in air, the thickness was measuredto be 18 microns.

The resulting solid polymer electrolyte was made into an MEA asdescribed above, and tested in a fuel cell using Test Protocol 2. Theresults (Table 7) show that the inventive solid polymer electrolyte ofExample 11-12 have about an order of magnitude lower fluoride releaserates than this Comparative Example. Further, when a strong solidpolymer electrolyte is used such as in Example 8-10, the observedLifetime was over three times longer, and the fluoride release rate anorder of magnitude lower than observed for Comparative Example 6.

TABLE 7 Fluoride Lifetime Release Rate Example No. (hr) (g/hr · cm²) Ex.8 582 1.30E−08 Ex. 9 637 8.38E−09 Ex. 10 586 2.46E−08 Ex. 11 <201.80E−08 Ex. 12 <31 1.90E−08 Comp. Ex. 6 184 1.30E−07

Example 13 and Comparative Example 7

The inventive materials contain a composite layer of a solid dispersioncomprising a plurality of support particles supporting a catalystcomprising a precious metal catalyst and an ion exchange material. Inthis example, this composite layer is shown to be substantiallyocclusive and electronically insulating. Two samples were prepared, acomposite layer of a solid dispersion of a plurality of carbon particlessupporting a platinum catalyst in an ion exchange material, and the sameion exchange material without the platinum supported on carbon. Thesetwo samples were prepared using ion exchange material Type 3 using thegeneral procedure outlined in Example 1. Here, the drawdown bar was setto 0.0254 cm (0.010 inches), the concentration of platinum in the inkwas 2.4% for Example 13, and no ink was used for Comparative Example 7.Example 13 was cast on a polyethylene terepthalate film whose surfacethat had been treated with a fluoropolymer to enhance release, whileComparative Example 7 was cast onto a glass plate. Only one pass wasmade, and no microporous film was applied. Both samples were heattreated at 160° C. for 3 min. Example 13 was removed from the film atroom temperature, while Comparative Example 7 was removed under roomtemperature water.

The catalyst-containing membrane layer, Example 13, is a physical modelof a composite layer of the invention, while Comparative Example 7 is alayer without catalyst. The latter is used herein to show that theproperties of the inventive composite layer are the same as a homogenouslayer without the platinum supported on carbon. To characterize theelectrical properties of the membrane-catalyst layer, electrochemicalimpedance measurements were conducted on the two membrane layers,Example 13 and Comparative Example 7: Fuel cell electrodes with aloading of 0.4 mg-Pt/cm² coated on a release layer were attached to bothsides of the membranes using 15 tons of pressure at 160° C. for 3minutes. The test was performed using the experimental proceduresdescribed by Johnson and Liu (ECS Proceedings Volume 2002-5, pages132-141). The impedance spectra were measured at a temperature of 80° C.and a relative humidity of 88% in an atmosphere of nitrogen gas. Theimpedance data for a frequency range of 20.0 kHz to 2.0 Hz are shown asa Nyquist plot (the imaginary vs. the real component of the impedance)in FIG. 11.

The impedance spectra for the two membranes are nearly identical,indicating the membranes have essentially the same electricalproperties. Furthermore, these spectra are characteristic of conicallyconductive membranes that are electronic insulators, i.e., there are notadequate pathways for electrons to pass through the membrane. Therefore,the composite layer of a solid dispersion comprising a plurality ofsupport particles supporting a catalyst comprising a precious metalcatalyst and an ion exchange material layer of the invention is anelectronic insulator.

A separate cast material prepared identically to that described abovefor Example 13 was tested using a standard Gurley air flow test. It hada Gurley value of greater than 10,000 s indicative of a substantiallyocclusive material. These tests taken together thus demonstrate that thecomposite layer comprising a plurality of carbon particles supporting acatalyst comprising platinum and an ion exchange material used in theinventive solid polymer electrolyte is both substantially occlusive andelectronically insulating.

Example 14

An additional inventive solid polymer electrolyte was prepared using theprocedures of Example 5 except the Type 3 ion exchange material wasmixed with ink to produce a solution that was 11.5% platinum weight todry ionomer (instead of 2.4% used in Example 5). This solution was notpassed through the Microfluidizer, but instead, a portion of it wasplaced in a 25 ml centrifuge tube, and subsequently centrifuged in anAdams Compact II Centrifuge (Beckton-Dickenson Inc., Franklin Lakes,N.J.) for about 20 min at 3200 rpm. After centrifuging, the supernatantwas used to prepare a solid polymer electrolyte as described in Example5. The final membrane had a very light grey color, a thickness of 15-18microns, and a measured platinum loading below the detection limit ofthe XRF (<0.001 mg/cm²).

Although several exemplary embodiments of the present invention havebeen described in detail above, those skilled in the art readilyappreciate that many modifications are possible without materiallydeparting from the novel teachings and advantages which are describedherein. Accordingly, all such modifications are intended to be includedwithin the scope of the present invention, as defined by the followingclaims.

We claim:
 1. A method of making a solid polymer electrolyte membranecomprising the steps of (a) preparing an ink solution comprising aprecious metal catalyst on a supporting particle and an ion exchangematerial; (b) providing a polymeric support having a microstructure ofmicropores; (c) impregnating said microstructure with said ink solutionto form a substantially air occlusive composite membrane.